Up until rather recently substantially all electronic keyboard musical instruments, generally electronic organs, have been of an analog nature. Some such organs have used direct or ac keying in which a note to be played is simply conducted by a mechanical key switch. Such note is either on or off, i.e., there is no possibility of attack and decay. Other organs have utilized indirect or dc keying in which the mechanical key switch has applied a potential to a controllable conducting device, typically a semiconductor such as a diode or transistor, although vacuum tube devices were occasionally used in an earlier day. It is possible to control the conductivity of such a device by the voltage level applied thereto. Thus, rc timing circuits of an analog nature have readily been able to control attack and decay.
With recent advances in electronics, and particularly large scale integrated (LSI) circuit chips, efforts have been made toward producing electronic musical instruments utilizing digital techniques, due to the facility of handling digital signals in LSI circuits and the difficulty of handling analog signals in such circuits. This has presented problems as to developing attack and decay, since the great advantage of digital techniques with integrated circuits is that a signal is at all times either a one or a zero. Hence, the signal cannot be scaled in magnitude as heretofore has been the case with dc keying in conventional analog organ circuits.